Zenith

The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The zenith is the "highest" point on the celestial sphere . This sense of the word is often used to describe the location of the Sun , but to an astronomer the sun does not have its own zenith, and is at the zenith only if it is directly overhead. In a scientific context, the zenith is the direction of reference for measuring the zenith angle, the angle between a direction of interest and the local zenith. The sun is at its zenith when it is 90 degrees above the horizon and this only happens between the Tropic of Cancer and the Tropic of Capricorn. If the sun is at its zenith at a location, it would be at its nadir at the antipode of that location. In astronomy, the altitude in the horizontal coordinate system and the zenith angle are complementary angles, with the horizon perpendicular to the zenith. The astronomical meridian is also determined by the zenith, and is defined as a circle on the celestial sphere that passes through the zenith, nadir, and the celestial poles. On the International Space Station, zenith and nadir are used instead of up and down, referring to directions within and around the station, relative to the earth.